Seasonal Prediction of Summer Monsoon Rainfall in

Central-East China

Contributed by Xu Qun

Jiangsu Meteorological Institute, Bei-Ji-Guo,No.2, Nanjing, 210008, China

According to the method of seasonal prediction of summer monsoon rainfall in central-east China developed by Xu and Yang (1997), seasonal prediction for summer 1999 are made by using the regression equations trained on data from a 20-year period:1979-1998. The predictors are selected from a series of high correlation regions of the following fields for January-March preceding the summer monsoon.

• Monthly 500mb height fields of the Northern Hemisphere for 576 grid points (10-85N), 1979-1998.

• Monthly sea surface temperature (SST) of the North Pacific for 286 grid points, 1979-1998.



• Monthly temperature for 160 stations evenly distributed throughout China, 1979-1998.

• Monthly precipitation for 160 stations evenly distributed throughout China, 1979-1998.

The selected predictors were then combined to form multivariate linear equations. We have found that a 20-year time series is needed for training the regression equations in order to minimize the effect of interdecadal variability and climate change. The process for selecting the predictors and predictands was described in June 1997 issue of this Bulletin. The predictors selected from above data fields show that the combination of some anomalies existed in preceding atmospheric circulation- sea surface temperature of the North Pacific and the land surface of China may play a key role in forming the anomalous distribution of summer monsoon rain in China with the lag of a season. The above normal trend of summer monsoon rainfall in the Yangtze-Huaihe basins of 1998, especially the large flooding trend in the Yangtze basin during June-July 1998 have been correctly predicted by this method (see the June 1998 issue of this Bulletin).

Prediction for Summer 1999

The following predictands were calculated by using the newly established regression equations and the forecast was made on April 25,1999. The first 9 predictands listed below were shown in the forecast issued in 1998 (see the June 1998 issue of this Bulletin). 5 additional predictands (Huai8r, Yan8r, Yanlsr, Yanmsr, CL68r) using the same training period are also presented here.

• Yanr68: Summer average rainfall anomaly in 10 stations in the mid-lower Yangtze basin (29.2-32.2N, east of 113E):+314.2 (Flooding).

• Meiyu: Percentage of total rainfall in 5 stations located in the mid-lower Yangtze basin during the period of Meiyu:160%.

• Onsemei: Date of onset of the Meiyu, which will come between the first and second ten days of June 1999 (Early).



• Outmei: The first day after the end of Meiyu, which will be postphoned to late July (Late).



• Snmei: Percentage of total rainfall in 5 stations located in the southern part of Jiangsu province during the Meiyu period: 153%.



• Jnmei: Percentage of total rainfall in 5 stations located in the middle part of Jiangsu province during the Meiyu period: 150%.

• Huai67r: The average rainfall anomaly of 8 stations evenly distributed in Huaihe basin (32-34N, east of 112E) during June-July: +192.6 (Flooding).



• Nor78r: Averaged summer (July-August) rainfall anomaly percentage in 10 stations evenly distributed in North China (36-41N, east of 110E: -19.4% (Drought).



• Jrs67r: Averaged rainfall anomaly in 7 stations evenly distributed in Jiannan region, south of the mid-lower Yangtze (27-29N,east of 112E): +236.8 (Above normal).



• Huai8r: The averaged rainfall anomaly of 8 stations evenly distributed in Huaihe basin during August: -67.4 (Below normal).



• Yan8r: The averaged rainfall anomaly in 10 stations evenly distributed in Yangtze basin during August: +26.9 (Normal).



• Yanlsr: Summer averaged rainfall anomaly in 4 stations in the lower Yangtze basin (31.0-32.8N, east of 117E): +314.2 (Flooding).



• Yanmsr: Summer averaged rainfall anomaly in 4 stations in the middle Yangtze basin (29-31N,111.5-117.0E): +380.9 (Flooding).



• CL68r: The central latitude of positive anomaly of summer monsoon rainfall in eastern China (east of 112E):32.0N (in the southern side of Yangtze-Huaihe basins).

The forecast calls for both an early onset and a later ending of the Meiyu in 1999. The central latitude of the summer monsoon rainy belt in eastern China is forecasted south than normal, however it will be north of its position during last summer by 3^o-4^o of latitude possibly due to the transition from the last El-Niño phase to present La-Niña event. It is notable that the Yangtze-Huaihe basins (especially the mid-lower Yangtze) will be flooded again after the great 1998 Yangtze flooding. Large positive anomalies of Meiyu rainfall are forecasted in the middle and southern parts of Jiangsu province with the threat of possible summer flooding from the upper reaches of Yangtze and Huaihe River. This trend of positive rainfall anomalies in the Yangtze and Huaihe basins are forecasted to continue until the end of July. The summer monsoon rain in North China will be less than normal. In short, the forecast calls for considerable flooding in the Yangtze-Huaihe basin during June-July 1999 with summer drought occurring in North China, however the central latitude of summer flooding will be north of its position compared to last summer by about 3^o-4^o latitudes, the flooding extent will be not so serious as in 1998.

References

Xu Qun and Yang Qiuming, 1997: Seasonal prediction of summer monsoon rainfall in central-east China. Experimental Long-Lead Forecast Bulletin, 6(2), 74-79.

Xu Qun, 1998: Seasonal predictions of summer monsoon rainfall in central-east China. Experimental Long-Lead Forecast Bulletin, 7(2), 79-80.